A thermoplastic resin, for example, a polyester resin as represented by polyethylene terephthalate (PET) excels in such properties as formability, transparency, mechanical strength and resistance against chemicals as well as relatively high barrier property against gases such as oxygen and the like, and has been used as a packaging material such as of film, sheet, bottle, etc. in a variety of fields. In order to improve gas-barrier property of the packaging material, further, there has been known a multilayer structure comprising a layer of a gas-barrier resin rich in gas-barrier property, such as a saponified product of ethylene-vinyl acetate copolymer or a polyamide as an intermediate layer sandwiched between an inner layer and an outer layer of a polyester resin via a suitable adhesive resin layer.
From the standpoint of saving resources and decreasing the weight of the packaging containers such as polyester bottles that have been placed in the market, attempts have been made to further decrease the thickness of the body portions. To meet the above requirement, further, it becomes necessary to suppress a decrease in the barrier property against the gases such as oxygen and the like caused by a reduction in the thickness, as a matter of course. In an embodiment that uses the gas-barrier resin, in this case, the container must be formed in a multiplicity of layers to shut off the permeation of gases making it difficult to decrease the thickness of the container to a sufficient degree.
Oxygen-barrier property can be improved by using an inorganic oxygen absorber such as iron powder. The oxygen absorber by itself undergoes oxidation to absorb oxygen; i.e., exhibits barrier property to shut off the permeation of oxygen upon absorbing oxygen causing, however, the resins to be colored. Therefore, the inorganic oxygen absorber is not used in the field of packaging where transparency is required. In the field of packaging, therefore, it is a general practice to use an organic oxygen absorber that does not cause resins to be colored.
A patent document 1, for example, proposes an oxygen-absorbing resin composition containing an oxidizing organic component (organic oxygen absorber) such as unmodified polybutadiene or maleic anhydride-modified polybutadiene.
Further, a patent document 2 proposes an oxygen-trapping composition containing a compound having an unsaturated alicyclic structure (cyclohexene structure) as an organic oxygen absorber.
For being oxidized, however, the above organic oxygen absorber requires a transition metal catalyst (e.g., cobalt or the like) which causes a variety of inconveniences. For example, the resin which is the base material is oxidized and deteriorated, too, permitting oxygen to permeate through the wall of the base resin and without, therefore, much improving barrier property against oxygen. Besides, the base resin that is oxidized and deteriorated brings about a decrease in the strength, too. Moreover, low-molecular decomposed products such as aldehyde and ketone are by-produced much, arousing such problems as the generation of offensive odor and a decrease in the property for retaining flavor of the contents. In the field of packaging, in particular, a decrease in the property for retaining flavor of the contents is a serious problem. When an organic oxygen absorber is used, therefore, it becomes necessary to employ a layer structure of which the resin layer blended with the organic oxygen absorber does not come in contact with the content in the container, i.e., to employ a multilayer structure. Therefore, the above-mentioned means is not suited for decreasing the thickness of the container wall.
A patent document 3 proposes a resin composition containing a resin that exhibits excellent oxygen-absorbing capability even under a condition where no transition metal catalyst is present. This proposal was made by the present applicant. Namely, the resin composition contains, as the oxygen-absorbing resin, a resin that contains a constituent unit derived from a compound having an unsaturated alicyclic structure, such as a Δ3-tetrahydrophthalic acid derivative obtained by the Diels-Alder reaction of a maleic anhydride with the diene. The oxygen-absorbing resin of this type is very reactive with oxygen, and not only exhibits excellent oxygen-absorbing capability even under a condition where no transition catalyst is present but also by-produces no low molecular decomposition product that becomes a cause of offensive odor. Therefore, the resin can be applied as a single-layer structure to the containers offering an advantage of realizing the containers having effectively reduced thickness and weight.
Here, upon adjusting the copolymer composition, the resin composition of the patent document 3 can be rendered to have a glass transition temperature which is lower than room temperature to improve oxygen-absorbing capability. If the above oxygen-absorbing structure is contained in a resin having a high glass transition temperature that is used for packaging containers, e.g., is contained in a polyester resin such as PET, however, it is not allowed to improve the oxygen-absorbing capability to a sufficient degree. That is, the above oxygen-absorbing resin has a glass transition temperature of −8° C. to 15° C. and in which the mobility of the molecules is very high in an atmosphere of room temperature; i.e., this mobility is one of the factors that produce excellent oxygen-absorbing capability. On the other hand, the polyester resin such as PET used in the field of packaging containers has a glass transition temperature of about 70° C. and, therefore, its mobility at room temperature is very low. Therefore, even if the above oxygen-absorbing resin having a low glass transition temperature is made present together with the polyester resin at room temperature, mobility of the molecules remains suppressed and, as a result, oxygen-absorbing capability cannot be produced to a sufficient degree. Further, the oxygen-absorbing resin composition having a low glass transition temperature makes it difficult to maintain the shape of the containers or the rigidity thereof. When used for forming the containers, therefore, the oxygen-absorbing resin composition is not used alone but is used together with other resins to form the containers in a composite multilayer structure. In fact, the patent document 3 is forwarding the study for realizing the composite multilayer containers but is not studying the realization of polyester containers comprising chiefly the PET.
Further, a patent document 4 proposed by the present applicant is disclosing an oxygen-absorbing resin composition of a polyolefin resin (A) obtained by polymerizing an olefin having 2 to 8 carbon atoms, blended with a resin (B) that works to trigger the oxidation of the resin (A) and a transition metal catalyst (C), and teaches the use of a styrenic polymer as the resin (B).
However, the above resin composition, too, necessitates the use of the transition metal catalyst, and is used for imparting oxygen-absorbing capability to the olefin resin, but is not applied to the polyester resin.
As described above, there has not been known yet any oxygen absorber that exhibits excellent oxygen-absorbing capability upon being added to a polyester resin (specifically, to a polyester resin of the packaging grade) without using transition metal catalyst.